Material processing

ABSTRACT

A method of materials processing (e.g. polymer extrusion) in which the material flows along an axially extending passageway. A pressure differential is established across the passageway transverse to the axial flow direction so as to create a flow of material through the die which has a velocity component transverse to the axial flow direction. Preferably the pressure differential transverse to the extrusion direction is continuous thereby estabilshing a continuous change of velocity component transverse to the axial flow direction. Preferably also there is at least one reversal of transverse pressure differential (and hence transverse velocity component). Apparatus for performing this method comprises for example, successive sections (3-5) of identical trapezoidal cross section with adjacent ones of these sections having their cross-sections displaced through 180° relative to each other.

The present invention relates to materials processing, particularly butnot exclusively processing of polymer melts (e.g. by extrusion).

The extrusion technique is well established for the production ofplastics articles, e.g. sheets and pipes, but does have certaindisadvantages associated with the flow pattern of the molten material inthe extrusion die. The production of discontinuous fiber reinforcedplastics sheet may be cited as an example. During production of suchsheets, the flow pattern of the molten plastics material results in thefibers becoming more or less uniaxially aligned in the direction ofextrusion. The resultant sheet is substantially anisotropic.

A further example is the production of tubular material. In this case,the flow pattern tends to align reinforcing fibers parallel to the tubeaxis whereas it may be desirable that the fibers be aligned at variousangles to this axis so as to increase the hoop strength.

An additional problem arises in the production of pipes (whetherreinforced or not) due to the formation of so-called weld lines. Theselines result from the struts (or `spiders`) used to hold the centralcore of the die in position. Molten material must flow around thesestruts resulting at the downstream side thereof in weld lines (or join)where the separated flow lines again meet. The resultant pipe includes anumber (depending on the number of struts) of axially parallel weldlines which may or may not be visible but which represent weaknesses inthe pipe.

Finally, in the production of sheets and pipes as discussed above, itmay be desired to control the molecular orientation within the product(e.g. to give multiaxial, orientation) and such control can be difficultto achieve using the conventional apparatus.

It is an object of the present invention to obviate or mitigate theabovementioned disadvantages.

According to a first aspect of the present invention there is provided amethod of extruding a solidifiable material through an extrusion diewherein a pressure differential is established across the die transverseto the extrusion direction thereby to create a flow of material throughthe die having a velocity component transverse to the extrusiondirection.

Preferably the pressure differential transverse to the extrusiondirection is continuous thereby establishing a continuous change ofvelocity component transverse to the extrusion direction.

Preferably also there is at least one reversal of transverse pressuredifferential (and hence transverse velocity component) during theextrusion of the material.

According to a second aspect of the present invention there is providedan extrusion die having an axially extending extrusion channel wherein,in its cross-sectional plane transverse to the flow direction, thechannel is shaped to establish a pressure differential transverse to theaxial flow direction. This shaping is preferably such that, in the saidcross-sectional plane, the depth of the channel varies in a directiontransverse to the axial flow direction.

In addition to the extrusion methods and apparatus defined in the twopreceding paragraphs, the invention also provides other methods of, andapparatus for, materials processing which embody the same principles.

The invention will be further described by way of example only withreference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic plan view of one embodiment of extrusionapparatus in accordance with the invention for producing sheet material;

FIGS. 2a-2e are sectional views on the line a--a, b--b, c--c, d--d, ande--e of the apparatus shown in FIG. 1;

FIG. 3 is a detail of FIG. 1;

FIG. 4 is a part sectional view of one embodiment of extrusion die forproducing a pipe;

FIG. 5 is a sectional view of the line X--X of the die shown in FIG. 4;

FIG. 6 is a part sectional view of a further embodiment of die; and

FIGS. 7a-c are sectional views on the lines A--A, B--B and C--C of FIG.6.

Referring to FIG. 1, the extrusion apparatus 1 illustrated therein isintended for producing fiber-reinforced plastics sheet and comprises aninlet section 2 (fed by an extruder screw-not shown), a first diesection 3, a second die section 4, a third die section 5 and an outletdie section 6. FIGS. 2a-e respectively illustrate transversecross-sectional views (taken on the lines a--a, b--b, c--c, d--d ande--e of FIG. 1) of the die sections 2-6. It will be seen from FIGS. 2a-ethat each of the adjacent extruder sections is of differentcross-sectional configuration and the reasons for this will be decribedmore fully below. It should also be noted from FIG. 1 that between eachadjacent extruder section is a transition region Ti,j where i is thereference numeral allocated to the upstream section and j is thereference numeral allocated to the downstream one. Thus, the transitionregion between inlet section 2 and first die-section 3 is referenced asT₂,3. These transition regions, Ti,j (as their name suggests) are shapedto provide a continuous transition from the cross-sectional shape ofsection i to that of section j. The regions Ti,j are as short aspossible in length without providing such an abrupt transition thatthere would be stagnation points at which molten material passingthrough the extrusion apparatus would accumulate.

Each of sections 2 to 6 will now be described in more detail.

Inlet section 2 is illustrated as being of rectangular cross-section asviewed along line a--a (FIG. 2a) and diverges from the extruder screw(not shown) to the first die section 3. The inlet section need not be ofrectangular section along its length and may be of the type referred toin the industry as a coat-hanger die.

First die section 3 is of the constant trapezoidal section shown in FIG.2b along its length between transition regions T₂,3 and T₃,4 and will beseen (as viewed in FIG. 1) to be of greater depth at its right hand side3r than at its left hand side 3l.

Second die section 4 is also of constant trapezoidal cross-section (FIG.2c) along its length between transition regions T₃,4 and T₄,5. Thiscross-section is dimensionally the same as that of die section 3 but isturned through 180° relative thereto (as will be appreciated from acomparison of FIGS. 2b and 2c). In other words, die section 4 is deeperat its left hand side 4l (as viewed in FIG. 1) than at its right 4r.

Third die section 5 is of the same cross section and orientation as thatof die section 3.

Outlet section 6 is of constant rectangular cross-section throughout itslength and, as will be described in more detail below, serves to ensurethat the extruded sheet material exiting from apparatus 1 has a flatvelocity profile to prevent warping of the extrudate.

In use of the apparatus, molten plastics material containing fiberreinforcement is supplied from the extruder screw (not shown) into inletsection 2 and then successively through sections 3-5 along a line ofpressure drop P₁ -P₂ (P₁ P₂) before emerging from the apparatus asextruded sheet material.

The flow of material in each of sections 2 to 4 is influenced by theflow in the adjacent downstream section. Consider, for example, section3. Owing to the fact that its trapezoidal cross-section is displaced by180° relative to that of the immediately downstream section 4, apressure differential is established across section 3 such that thepressure P₃ at the right hand side 3r (the deeper side) is less than thepressure P₄ at the left hand side 3l (the shallow side). A velocitygradient is therefore established across section 3 in the oppositedirection to the pressure gradient (i.e. V₁ V₂). At any point withinsection 3, the molten material will tend to flow in the direction of avelocity vector V_(R) which is the resultant of the axial velocitycomponent V_(A) and the transverse velocity component V_(T) at thatpoint. These velocity components V_(A) and V_(T) vary along the lengthand across the breadth of section 3 so that the magnitude and directionof V_(R) changes throughout the section. For example, the axial velocitycomponent V_(A) at the upstream end of side 3l is less than at theupstream end of side 3r whereas the opposite is true for the transversecomponents V_(T) at these points. It is these variations which influencethe flow in section 2 (in the manner detailed below). Moreover, thevalue of V_(A) increase in the downstream direction along side 3lwhereas the opposite is true along side 3r. This variation is due to theeffect imposed by the opposite orientation of section 4 (vis a vissection 3) which results in pressure and velocity gradients thereinwhich are in the opposite directions to those in section 3. It shouldalso be mentioned that, within section 4, the flow is influenced by thatwithin section 5 (owing to the different orientation of the respectivetrapezoidal cross-section). As a further point it should be noted that(as will be appreciated from the subsequent description) neither ofsections 3-5 are of such length that steady state fiber orientationconditions are established therein.

The overall effect is that, in any cross-sectional plane of theapparatus, there is a maximum value of V_(R) (which may not be the samealong the length of the apparatus or indeed along any one section) whichchanges along the axial length of the apparatus. This is illustrated bythe line 6 shown in FIG. 1 which is the line along which the maximumvalue of V_(R) for any transverse sectional plane of the apparatus is tobe found.

Referring back now to section 3, it was explained above that themagnitude and direction of V_(R) changes throughout the section. At anypoint within the section, a fiber 8 will, depending on the magnitude ofV_(R) at that point tend to be aligned to a greater or lesser extentalong the direction of V_(R). However, as the fiber moves throughsection 3, the magnitude and direction of V_(R) changes with the resultthat the fiber 8 is continuously being reoriented. It should be notedthat, owing to the influence of the flow in section 3 on that in section2, this orientation of the fiber 8 begins in this latter section ratherthan when it enters section 3.

At the downstream end of section 3, the orientation of fibers 8 (beforeentry into section 4) will typically be as shown in FIG. 3 and it willbe seen that these fibers make a range of angles with respect to theline P₁ -P₂. For any particular fiber 8 in section 3 its leading end xwill be in a velocity stream moving more quickly (albeit only slightly)than that in which its trailing end y locates (due to the velocitygradient across section 3).

As the fiber passes transition section T₃,4, it enters die section 4 inwhich the velocity gradient is reversed as compared to section 3. Withinthis section, the fiber 8 is also subjected to continuously changingvalues in the magnitude and direction of V_(R) to cause reorientation ofthe fiber. In more detail, the end x of the fiber which, in section 3,formed the leading end and was in a higher velocity stream than thetrailing end is now in a lower velocity stream than end y. The result isthat end y is accelerated forwards relative to end x to reorientate thefiber.

It will be recalled that the fibers in section 3 are oriented atdifferent angles to the line P₁ -P₂ and consequently enter section 4 atdifferent angles. The extent to which a fiber is reoriented in section 4depends on its angle of entry thereto together with the viscosity of themolten material and the residence time in section 4. Since the fibers indie section 3 are oriented at different angles, their resultantreorientations in die section 4 will be different with the result that,in this section, the fibers will become randomly oriented. In fact,fibers in section 4 will criss-cross each other to produce a mattingeffect. Moreover since steady state fiber orientation conditions are notestablished in die section 4, the fibers maintain their randomorientation as they enter outlet section 6. The purpose of this sectionis merely to establish rectangular cross-section for the extrudate (toprevent warping as the material exits the apparatus). However the lengthof section 5 is not so long that the fibers reorient themselves alongthe flow direction.

The resultant sheet is planar isotropic which results from the fact thatthe fibers are more or less randomly oriented within the solidifiedplastics material.

Additionally the flow conditions within the apparatus ensure amulti-axial orientation of the polymer molecules.

It will of course be appreciated that the illustrated apparatus may beused for the production of non-reinforced sheet material.

It should be appreciated that a number of modifications may be made tothe illustrated extrusion apparatus. Thus, for example, only one of eachof sections 3 and 4 need be provided. Alternatively two or more of eachof sections 3 and 4 may be provided in alternating relationships.Additionally, the section 3 need not be of the same cross-section (whenconsidered after rotation of one of these cross-sections through 180°)nor do the individual sections need to be of the same length.Furthermore, at least the section immediately upstream of the outletsection 6 may increase in depth along its axial length to assist inachieving a flat velocity profile in section 6. Finally, sections 3 and4 may have a transverse cross-section other than that illustrated so asto assist in the production of sheet material of rectangularcross-section from particular polymeric materials.

It should be appreciated that with all of the possible variations whichmay be made it is possible to engineer the extrusion apparatus toproduce any desired properties for the sheet material, e.g. degree ofmolecular orientation, degree of fiber orientation etc.

FIGS. 4 and 5 illustrate one embodiment of extrusion die for producingpipes by using the principle of the invention. The illustrated diecomprises on outer, circular section housing 10 in which a core member11 is supported with clearance by spiders 12. The core member 11 has,over that section of its length designated as 11a, a cross-section asillustrated in FIG. 5. This cross-section is akin to one turn of aspiral such that the surface of core-member 11 progressively increasesin distance from the inner surface of the housing 10 going (in aclockwise direction as viewed in FIG. 5) from its point a of closestapproach thereto to its most distant point b therefrom. A transitionregion T_(a),b extend between points a and b. The section of core member11 designated in FIG. 4 as 11b has a transverse section identical tothat of section 11a but displaced through 180°. The cross-section of 11cis identical to, and of the same orientation as, that of 11b.

Although not illustrated in the drawing the extrusion die willpreferably have an outlet section to ensure that a pipe of uniform wallthickness is produced.

In use of the illustrated die, a pressure differential is establishedaround the core member 11 such that the pressure decreases from amaximum at point a to a minimum at point b. A velocity gradient is thusestablished around core-member 11, in the same way that one wasestablished across die sections 3 and 4 of the apparatus illustrated inFIG. 1.

The pressure differential results in molten material which is suppliedto the direction of arrow 13 having a velocity component V_(T)transverse to the axial flow direction along which the material has avelocity component V_(A). The resultant velocity vector is shown asV_(R). The value of V_(R) varies around the periphery of core member 11(due to the cross section thereof). So, as in the case of the apparatusshown in FIG. 1, fibers incorporated in the material will be subject tocontinuous reorientation. Overall the material follows a helical patharound section 11a the length of which may be such that the materialexecutes a single turn of a helix or only part thereof. As explainedabove, fibers incorporated in the molten material are continuouslyreoriented so as to be inclined at various angles to the tube axis, thusimproving the hoop stress of the resultant tube.

As mentioned above, core 11 is supported by spiders 12 which tend tolead to the production of axially parallel weld lines due to the needfor the molten material to flow around the spiders 12. In order toeliminate these weld lines it is beneficial to establish a velocitygradient through the thickness of the molten material in the transversedirection. To this end, the section of core member 11 designated in FIG.4 as 11b has a transverse section identical to that of section 11a butdisplaced through 180° such that there is a step formation asillustrated between the two sections. This step formation results in ahigh velocity gradient through the thickness of the material in thetransverse direction.

Thus, three mutually perpendicular velocity components are present, andthe resultant movement of the material causes the elimination of weldlines.

The apparatus shown in FIG. 4 has been described with specific referenceto fiber reinforced tubes. However it will be appreciated that the flowpatterns established within the die also affect the molecularorientation within the material.

Having regard to the above description it will be appreciated that theapparatus illustrated in FIG. 4 may be (as in the case of thatillustrated in FIG. 1) engineered to such particular requirements. Thus,for example, the apparatus may have two comparatively short inletsections separated by step portions for the elimination of weld linesfollowed by two longer sections shaped and dimensioned to ensure biaxialmolecular orientation within the product.

FIGS. 6 and 7 show a further embodiment of die for producing a pipe. Inthis case the die comprises a circular section housing 20 with acircular section core-member 21. This core member is supported (e.g. byspiders not shown) such that its longitudinal axis is angled withrespect to, and at its center intersects with, the longitudinal axis ofthe housing 20.

At the said point of intersection of the axes, the cross-section ofcore-member 21 is concentric with that of housing 20 (FIG. 7B) whereas,on either side of the intersection, these sections are eccentric (FIGS.7A and 7C). Owing to the fact that the eccentricities upstream of thepoint of intersection of the axes are displaced by 180° relative tothose downstream of this point, pressure gradients are establishedaround the core-member 11 resulting in a velocity gradient therearound.To the left (as viewed in FIG. 7) of the intersection point, thisvelocity has components represented by arrows 22 whereas, to the rightof the intersection points the velocity gradients are in the oppositedirection as represented by arrows 23. Consequently, in this embodiment,the molten material is subjected to a reversal of flow, much the sameway as occurs in the apparatus illustrated in FIG. 1.

A further embodiment of die (not-shown) for extruding as pipe inaccordance with the principle of the invention would comprise a circularhousing having a central core member constructed as a series ofidentical circular discs mounted in adjacent face-to-face relationshipwith their centers on a helical locus.

Although the dies illustrated in FIGS. 4-7 (and the die mentioned in thepreceding paragraph) have been described with specific reference to theproduction of tubes as the end product, it should be appreciated thatthese dies may also be used for the production of tubular parisons whichare subsequently blow-molded to produce the finished product.

It should also be appreciated that the invention may be used to producea wide range of extruded products, e.g. biaxially or multiaxially ororiented films.

Finally, it should also be appreciated that the principals embodied inthe present extrusion method may also be used in other techniques, suchas injection molding.

I claim:
 1. A method of materials processing in which the material flowsalong an axially extending passageway with successive upstream anddownstream sections wherein the cross-section (as viewed in the planetransverse to the flow direction) of the inlet to the downstream sectionhas first and second end regions between which the length (as viewed insaid cross-section) of the channel extends and this cross-sectionincreases progressively in depth from the first end region to the secondend region thereof, and said first and second end regions arerespectively of lesser and greater depth than the adjacent end region ofthe outlet of the upstream section, whereby a pressure differential isestablished in said sections so as to create therein a flow of materialhaving a velocity component transverse to the flow direction.
 2. Amethod as claimed in claim 1 wherein the axially extending passageway isan extrusion die.
 3. A method as claimed in claim 2 wherein saidpressure differential is continuous thereby establishing a continuouschange of velocity component without any disruption of the flow in saidsection transverse to the axial flow direction.
 4. A method as claimedin claim 1 wherein the cross-section of the outlet of upstream sectionincreases progressively in depth from a first end to a second end regionthereof and the inlet to the downstream section has the samecross-section as that of the outlet to upstream section but displacedthrough 180°.
 5. A method as claimed in claim 4 wherein thecross-sections of said upstream and downstream sections are constantover at least a part of their axial lengths.
 6. A method as claimed inclaim 5 wherein each said cross-section is trapezoid.
 7. A method asclaimed in claim 5 wherein the upstream and downstream sections aregenerally annular with one boundary in the form of one turn of a spiralas viewed in the plane transverse to flow direction so as to give saidprogressive increase in depth within said inlet and outlet ends.
 8. Amethod as claimed in claim 1 wherein each said cross-section variesalong the axial length of the section.
 9. A method as claimed in claim 1wherein said material is a molten plastics material.
 10. A method asclaimed in claim 1 wherein said material includes a fibrous filler. 11.Material processing apparatus having an axially extending flowpassageway with successive upstream and downstream sections wherein thecross-section (as viewed in the plane transverse to the flow direction)of the inlet to the downstream section has first and second end regionsbetween which the length (as viewed in said cross-section) of thechannel extends and this cross-section increases progressively in depthfrom the first end region thereof to the second end region thereof, andsaid first and second end regions are respectively of lesser and greaterdepth than the adjacent end regions of the outlet of the upstreamsection whereby, in use of the apparatus, there is established in thedownstream section a pressure differential transverse to the axial flowdirection so as to create a material with a velocity componenttransverse to the flow direction.
 12. Apparatus as claimed in claim 11wherein the cross-sections of the upstream and downstream sections areshaped to establish a continuous change of pressure differentialtransverse to the flow direction in said one downstream.
 13. Apparatusas claimed in claim 11 which is an extrusion die.
 14. Apparatus asclaimed in claim 11 wherein the outlet of the upstream section has across-section increasing in depth from a first end region to a secondregion thereof and the inlet of the downstream section has an identicalcross-section but displaced through 180° relative to that of the outletof the upstream section.
 15. Apparatus as claimed in claim 14 whereinthe cross-section of each of said upstream and downstream sections isconstant over at least a part of the axial length of the respectivesection.
 16. Apparatus as claimed in claim 15 wherein each saidcross-section is trapezoid.
 17. Apparatus as claimed in claim 15 whereineach said section is generally annular with one boundary in the form ofone turn of a spiral.
 18. Apparatus as claimed in claim 14 wherein saidcross-section of each said upstream and downstream sections varies alongtheir axial lengths.
 19. Apparatus as claimed in claim 11 comprising anoutlet section shaped such that, in use of the apparatus, there issubstantially no velocity differential transverse to the axial flowdirection in the outlet section.
 20. Apparatus as claimed in claim 11comprising in succession in the flow direction an inlet section, a saidupstream section, a said downstream section, the cross-sections of saidupstream and downstream sections being such that the pressuredifferential established in the latter is a reversal of that in theformer, and an outlet section.
 21. Apparatus as claimed in claim 20wherein said upstream and downstream sections have cross-sections in theform of identical trapezoids with that of the upstream section being at180° relative to that of the downstream section.
 22. Apparatus asclaimed in claim 11 wherein said flow passageway is defined by acylindrical bore, and the upstream section has located therein withclearance a former with a boundary in the form of one turn of a spiralas viewed in the plane transverse to the flow direction there being atransition region between the ends of the spiral.
 23. Apparatus asclaimed in claim 11 wherein said flow passageway is defined by acylindrical bore, and said downstream section of said flow passagewayhas located therein with clearance a former with a boundary in the formof one turn of a spiral as viewed in the plane transverse to the flowdirection there being a transition region between the ends of thespiral.
 24. Apparatus as claimed in claim 23 wherein said upstreamsection has located therein with clearance a former with a boundary inthe form of one turn of a spiral as viewed in the plane transverse tothe flow direction there being a transition region between the ends ofthe spiral, the former of the upstream section being displaced by 180°relative to that of the downstream section.